Define a struct inside a class in C++

Something like this:

class Class {
    // visibility will default to private unless you specify it
    struct Struct {
        //specify members here;
    };
};

declare class & nested struct probably in some header file

class C {
    // struct will be private without `public:` keyword
    struct S {
        // members will be public without `private:` keyword
        int sa;
        void func();
    };
    void func(S s);
};

if you want to separate the implementation/definition, maybe in some CPP file

void C::func(S s) {
    // implementation here
}
void C::S::func() { // <= note that you need the `full path` to the function
    // implementation here
}

if you want to inline the implementation, other answers will do fine.